This invention relates to an electronic circuit embedded into the surface of a three-dimensional molded object and a method for making the same. More particularly, the present invention relates to a plane conformable precise metallic pattern, such as an electronic circuit, in intimate contiguous relationship with a three-dimensional hard plastic object and a method for making the same.
Circuit elements, such as microwave antennas, are often desired to be placed on curved, three dimensional plastic surfaces. To meet this need, the circuit element has to be curved to conform with the curvature of the surface. An example of curved circuitry is a patch or spiral microwave antenna described in U.S. Pat. Nos. 4,051,480 and 4,010,470 to Jones. These patents demonstrate the utility of conformal antennas.
Microwave antenna circuit elements must also have high accuracy, (on the order of a fraction of several thousandths of an inch), for the circuit to function properly. However, it is difficult and expensive to form highly accurate metallic patterns or circuit elements on a curved surface compared to forming similar circuits on a flat surface. This difficulty and expense limits the use of curved circuit elements regardless of their advantages.
What applies to microwave circuit elements applies more broadly to other high speed signal transmission such as high frequency analog or short pulse signals used increasingly in digital and telecommunication applications. In these cases, as in microwave applications, great demands are put on electrical impedance contributions of the dielectric supports of circuit traces and fine accurate dimensions of the conductive traces themselves. It is also increasingly understood that special advantages accrue to curved 3-dimensional circuitry in such applications, since improved use of space, economies of production and shorter distances for signal propagation become possible.
In recent years, several new approaches have been taken to attach conductive Patterns onto curved plastic surfaces. In one approach, an already molded part having a plane conformable surface is pressed against a carrying sheet. The carrying sheet has a circuit pattern on its surface which is comprised of conductive particles in a resinous matrix, on its surface. Upon application of heat, the resin adheres to the rigid plastic substrate, and the conductive particles settle out to a more densely packed condition of enhanced conductivity. The resin is typically a thermoset which conforms to the curved surface of the molded article. Heating of the resin fixes the conductive particles to the substrate and thus defines the location of the pattern. Unfortunately, this method is deficient for microwave applications. The random nature of the aggregated particles create surface irregularities which can cause unwanted radiations of electromagnetic energy. Also, these conductors have a high resistivity which is unacceptable for use in a microwave or other high speed circuit. This method also requires a separate positioning and heating step in addition to a molding step for placement of the circuit onto the substrate which is inefficient and costly.
Another approach to producing three-dimensional metallic patterns on a plastic substrate makes use of a patterned mold cavity. In this method the circuit pattern exists as an electrically conductive region of the mold while the remainder of the mold is non conductive. Metallic copper is plated onto the conductive region. The copper adheres to the conductive regions of the mold, but does not adhere to the other regions of the mold. Excess copper is removed and plastic is forced into the mold. Steel or a similar metal is preferably used as the mold material, so that the copper adheres to the plastic during the molding step. This effects a transfer of the pattern to the plastic from the metal wall when the steel mold is removed. However, this prior art method also suffers from certain known problems. For example, this approach is inherently very expensive. It requires replating the mold with the copper pattern between each molding step, which is time consuming, or using preplated mold inserts, which entails expensive machining for each insert to achieve the high precision required. Also, many desirable molding compounds contain abrasive fillers which eventually degrade the pattern definition, resulting in poor quality and eventual replacement of the mold or inserts. Also, use of a distinct circuit pattern will require new inserts or a new mold thereby further increasing expense.
Yet another prior art approach involves molding in raised relief the desired circuit pattern using an easily electroplated filled plastic. Then, in a second molding step, non-plating plastic is molded in the depressed region of the original molded surface. Unfortunately, this method requires specially filled plastics in which the fillers are inimical to the needs of low dissipation factor and well controlled dielectric constant which are necessary properties for microwave and other high speed circuits substrates. In addition, relying on molding thin lines of conductor or material (later plated to be conductive) is not a reliable method for holding tight tolerances on narrow lines of critically controlled impedance.